1. Field of the Invention
The present invention relates to an oxygen electrode and a process for the preparation thereof. More particularly, the present invention relates to a small oxygen electrode fabricated by utilizing a micro-machining technique and a process for the preparation thereof. Furthermore, the present invention relates to a method of bonding a fluorine resin film onto a substrate. More particularly, the present invention relates to a method of bonding a fluorine resin film tightly to a silicon wafer, a glass substrate or the like in the field of the semiconductor process or micro-machining.
2. Description of the Related Art
Oxygen electrodes are advantageously used for the measurement of the dissolved oxygen concentration. For example, the biochemical oxygen demand (BOD) in water is measured from the viewpoint of maintenance of the water quality. An oxygen electrode can be used as a device for measuring this dissolved oxygen concentration. Furthermore, in the fermentation industry, in order to advance the alcoholic fermentation at a high efficiency, it is necessary to adjust the dissolved oxygen concentration in a fermenter, and a small oxygen electrode can be used as means for measuring this dissolved oxygen concentration. Moreover, a small disposable oxygen sensor is demanded in the medical field. Still further, a small oxygen electrode can be combined with an enzyme to construct an enzyme electrode, and this enzyme electrode can be used for measuring the concentration of a saccharide or an alcohol. For example, glucose reacts with dissolved oxygen in the presence of an enzyme called glucose oxidase and is oxidized to gluconolactone. By utilizing the phenomenon that the amount of dissolved oxygen diffused in an oxygen electrode cell is reduced by this reaction, the glucose concentration can be determined from the amount of dissolved oxygen consumed.
As is seen from the foregoing description, the small oxygen electrode can be used in various fields such as environmental instrumentation, the fermentation industry and clinical medical treatment, especially in a case where the small oxygen electrode is attached to a catheter and is inserted into the body. Since the size is small and the electrode is disposable and cheap, the utility value is very high.
Since the size cannot be reduced in commercially available oxygen electrodes and mass production is impossible, the present inventors developed a new small oxygen electrode fabricated by a lithographic technique and an anisotropic etching technique and filed a patent application for this oxygen electrode (U.S. Pat. No. 4,975,175 corresponding to Japanese Unexamined Patent Publication No. 63-238548, see FIGS. 7 through 9). The oxygen electrode of this type has a structure in which two electrodes, that is, an anode 4 and a cathode 5, are formed on a hole 2 formed on a silicon substrate 1 by anisotropic etching through an insulating film 3. An electrolyte-containing liquid 6 is contained in this hole and finally, the top surface of the hole is covered with a gas-permeable (membrane) film 7. In the drawings, reference numeral 8 represents a responding part and reference numeral 9 represents a pad. This small oxygen electrode is small in size and the dispersion of characteristics is small. Moreover, since mass production is possible, the manufacturing cost is low.
Thus, conventional small oxygen electrodes have been improved almost to a practically applicable level by making improvements to the materials used for the fabrication. However, there are still some unsolved problems for preparing small oxygen electrodes along a manufacturing line and marketing them, as described below.
(1) In many cases, it is difficult to selectively form an electrolyte layer and a gas-permeable layer. Accordingly, the number of operations conducted for each chip is increased and the productivity is reduced. Therefore, the price of the small oxygen electrode rises. PA1 (2) The operation of forming an electrode pattern from above the hole formed by anisotropic etching toward the bottom becomes difficult as the step depth increases, and the precision of formation of the pattern is reduced and special means such as lap baking becomes necessary. Accordingly, the fabrication is very troublesome. PA1 (3) In conventional small oxygen electrodes, since the gas-permeable film is directly formed on the responding part, an electrode infiltrated into a gel or a polymeric solid electrolyte is used. However, it is difficult to regulate precisely the quantity of the electrolyte and, as a result, the dispersion of characteristics is adversely influenced. PA1 (4) A gas-permeable film was formed at the outset by dip coating or spin coating a liquid material. Most of materials used for such gas-permeable films (silicone resins and the like) deteriorate over time because the storage stability per year cannot be guaranteed. PA1 (5) It is sufficient if the gas-permeable film is spread only in the vicinity of the working electrode, but in the above-mentioned oxygen electrode, where only one silicon substrate is used as the substrate, the gas-permeable film should also be formed in an irrelevant portion such as an anode region, and the gas-permeable film is readily damaged.
In the field of semiconductor processing or micro-machining, it is demanded that a fluorine resin film, for example, as a gas-permeable film or an insulating material, should be tightly bonded to a silicon wafer or a glass substrate, as in the case of a small Clark cell (barrier membrane type) fabricated by utilizing the micro-machining technique.
As previously pointed out, a small oxygen electrode can be used in the fields of environmental instrumentation, the fermentation industry and clinical medical treatment, especially in a case where the small oxygen electrode is attached to a catheter and is inserted into the body. Since the size is small and the electrode is disposable and cheap, the utility value is very high.
In the production of a small oxygen electrode, for example, a small oxygen electrode disclosed in U.S. Pat. No. 4,975,175 corresponding to Japanese Unexamined Patent Publication No. 63-238548, formation of a gas-permeable film is accomplished by forming a water-repellant polymer film by dip coating or spin coating, and in the latter case, bonding a fluorine type fluorinated ethylene propylene (FEP) film by heat fusion. The process disclosed in Japanese Unexamined Patent Publication No. 63-238548 is simple, but the process is defective in that it is generally difficult to reconcile the selective formation of a film pattern with an increase of the film strength.
In the conventional semiconductor process, there is known a method of bonding a fluorine resin as an insulating material to a silicon wafer or a glass substrate where fusion bonding is carried out at the fluorine resin-melting temperature (about 280.degree. C.). However, the film formed by this method is readily peeled by incorporation of bubbles at the fusion bonding, change of the temperature or friction, and the resistance to wetting with water is very low.
In the case where a biosensor such as a small sensor is used in the medical field, the sensor should be subjected to high-pressure vapor sterilization in advance. Peeling of the gas-permeable film is frequently caused at this high-pressure vapor sterilization, and this is a very serious practical problem.